More than 10 years ago, the US Food and Drug Administration approved a health claim for soyfoods and coronary heart disease based on the cholesterol lowering-effects of soy protein. This decision confirmed years of clinical research which showed that soy protein directly lowered blood cholesterol levels. Long before this decision, soyfoods were already thought of as heart-healthy. Although they are rich in high quality protein, soyfoods are low in the saturated fat that is present in many of the more commonly consumed sources of protein in the US diet. Most of the fat in soyfoods is polyunsaturated, fat, the type of fat that lowers blood cholesterol and they are also among the few plant sources of the essential omega-3 fat alpha-linolenic acid.
The public health importance of the FDA decision should not be underestimated. Cardiovascular disease, which affects about 17 million people in the United States, is still the number one killer of Americans. It is currently estimated that each year about 800,000 Americans have a new coronary attack and 500,000 suffer a recurrent one. Despite the enormous amount of research showing that soy protein lowers cholesterol, the American Heart Association (AHA) reversed their earlier position endorsing the health claim in 2006. In their decision, the AHA said that although soyfoods played a role in fighting heart disease because of their favorable fatty acid profile, the evidence no longer supports the claim that soy protein itself lowers blood cholesterol levels.
In their position paper, the AHA concluded that soy protein lowered cholesterol by about 3%, on the basis of 22 studies and that this was not enough in their opinion to justify the health claim. However, new research from the University of Toronto by David Jenkins, MD, PhD, and colleagues shows the AHA erred in their conclusion. Jenkins and his team conducted a formal statistical analysis, something the AHA did not do, of the 22 studies cited by the AHA, and found the estimate of the cholesterol-lowering effects of soy protein was almost 50% higher than what the AHA stated. Furthermore, when the analysis was restricted to the 11 higher-quality studies, it was estimated that soy protein lowered cholesterol 5.2 percent.
Part of the analysis also provided, for the first time, actual estimates of the extent to which substituting soyfoods for more conventional sources of protein in the US diet lowers blood cholesterol levels. Of course, the decrease depends upon the amount of soyfoods consumed and the specific foods they replace. When consuming about 25 g of soy protein daily – the amount established by the FDA as the amount needed to lower blood cholesterol – from a range of soyfoods, blood cholesterol was lowered by 4.3 percent. Thus, combining the direct or intrinsic effect of the protein with the intrinsic or indirect effect from the favorable fatty acid profile of soyfoods will, in theory, lower cholesterol by over 8 percent. Since each one percent reduction in cholesterol levels lowers heart disease risk by one to two percent, incorporating soyfoods into the diet can clearly lead to a substantial decrease in heart disease risk.